1. Field of the Invention
The present invention relates to a catalytic combustion device wherein fuel, together with combustion air, is supplied to a catalyst to cause an oxidative reaction, thereby conducting catalytic combustion to generate heat which is used as a heating source for a heating apparatus, drying apparatus, cooking apparatus, etc.
2. Description of the Related Art
A typical conventional catalytic combustion device comprises, as shown in FIG. 8, a liquid fuel tank 1, a fuel pump 2, a premixing chamber 3, a straightening vane 4, a catalyst 5, an air preheater 6 and a blower 7 [see, for example, Japanese Patent Laid-Open No. 61-134515 (1986)]. More specifically, the air preheater 6 is constituted by a preheating burner or an electric heater or a means which performs heat exchange with either of them. When the combustion device is started, either the preheating burner or the electric heater is used, and during steady-state combustion any of the three means is used. The liquid fuel is pressurized in the fuel pump 2 and atomized through an atomizer nozzle 8 before being introduced into the premixing chamber 3. On the other hand, combustion air is heated in the air preheater 6 before being introduced into the premixing chamber 3. In general, the premixing chamber 3 has a circular or rectangular cross-section which diverges downstream, that is, the cross-sectional area gradually increases toward the downstream side, and the premixing chamber 3 is connected at the downstream end thereof to the catalyst 5. The liquid fuel atomized through the atomizer nozzle 8 swirls together with the combustion air heated in the air preheater 6 to mix therewith while evaporating within the premixing chamber 3. The resulting fuel-air mixture passes through the straightening vane 4 to reach the catalyst 5 where it burns by a catalytic reaction to form a high-temperature combustion gas which is then released through the air preheater 6.
The above-described conventional catalytic combustion device suffers, however, from the following problems that must be solved in practical application. Since it needs the air preheater 6 in addition to the premixing chamber 3, the overall size of the device increases and the arrangement is complicated, which leads to an increase in the cost. Further, since the prior art is arranged such that the premixing chamber 3 is heated by means of the air preheated in the air preheater 6, when the air preheating temperature lowers, the wall temperature of the premixing chamber 3 also lowers, so that the fuel colliding with the wall of the premixing chamber 3 may adhere thereto without evaporating, resulting in unstable combustion. For this reason, the air must be preheated to high temperature at all times, which requires a large-sized air preheater and hence leads to an increase in the cost. In addition, since the temperatures of the air preheater 6, the premixing chamber 3 and the air piping that connects them together are high, a large amount of heat is lost from the outer walls of these members into the ambient atmosphere.
Another conventional catalytic combustion device has, as shown in FIG. 9, a combustion tube 11 which is connected to a vaporizer 10 provided with an electric heater 9 for preheating. The vaporizer 10 has a combustion air inlet passage 12 and a fuel supply pipe 13 provided at the outer end thereof. A fuel atomizing and diffusing means 14 is provided inside the vaporizer 10. A diffusing vane 15, a flame holder 16, a flame arrester 17 and a catalyst 18 are installed in the combustion tube 11 at the downstream side of the vaporizer 10. An igniter 19 is provided immediately behind the flame holder 16 [see, for example, Japanese Patent Laid-Open No. 59-12218 (1984)].
When this prior art device is started, the vaporizer 10 is first preheated by means of the electric heater 9. After the temperature of the vaporizer 10 has been raised to a sufficient level, fuel and combustion air are supplied to form a fuel-air mixture. Then, the igniter 19 is activated to form small flames along the outlets of small bores in the flame holder 16, thereby carrying out catalytic heating. After the catalyst temperature has risen sufficiently, the supply of fuel is temporarily suspended, or the supply of fuel and air is increased several times, to extinguish the flames, and then catalytic combustion is initiated.
This prior art, however, needs the electric heater 9 for heating the vaporizer 10. During catalytic combustion, fuel adheres to the flame holder 16, which results in unstable combustion. In addition, to initiate catalytic combustion, the flow rate of fuel or air must be changed and, at this time, the catalyst temperature suddenly changes. There is therefore a fear of the lifetime of the catalyst being shortened by the thermal shock. There is also a fear that, when the air flow rate lowers, back fire may occur, that is, flames may be formed again on the flame holder 16. Thus, the described prior art also has problems which must be solved in practical application.
Still another conventional catalytic combustion device is shown in FIG. 10. In this prior art, fuel and air which are premixed are supplied to a catalyst 81 where the mixture burns to make the catalyst 81 red hot. In this case, a premixing chamber 82 is provided at the upstream side of the catalyst 81, and an exhaust chamber 85 at the downstream side thereof. In order to allow the catalyst surface to become red hot uniformly, a pressure equalizer 84 is provided in the exhaust chamber 85 independently of the catalyst 81 so that the amount of gas flowing through the catalyst 81 is uniform. The pressure equalizer 84 has bores 83. Bores 83 which have a relatively small diameter are provided in that portion of the pressure equalizer 84 which is closer to the exhaust chamber 85, while bores 83 which have a relatively large diameter are provided in that portion of the pressure equalizer 84 which is remote from the exhaust chamber 85. This enables combustion gas to be uniformly supplied to the catalyst 81 [see, for example, Japanese Patent Laid-Open No. 61-161324 (1986)].
This prior art, however, also suffers from the following problems. Since the pressure equalizer 84 is provided, among the heat radiated from the catalyst 81, only the heat on the side of the premixing chamber 82 can be utilized. In addition, since the pressure equalizer 84 is placed in the high-temperature combustion gas atmosphere, it must have considerably high durability in practical application. Since the pressure equalizer 84 is disposed behind the catalyst 81 with a predetermined spacing provided therebetween, the conventional catalytic combustion device that has a relatively wide catalyst surface is readily affected by fluctuations of the gas flow rates in the premixing chamber 82 and the exhaust chamber 85, so that the flow rate of gas passing through the catalyst 81 is likely to lack uniformity for the whole area of the catalyst 81. For this reason, the temperature is not uniform throughout the catalyst 81 and local heating may invite deterioration of the catalyst.